Fore/aft looking airborne radar

ABSTRACT

An antenna system for an airborne radar system with a dorsal unit having two opposing long sides extending in a height direction (Z) and a longitudinal direction (X), and two opposing short sides extending in a lateral direction (Y) and the height direction (Z), and an upper side opposing a bottom side each extending in the longitudinal direction (X) and the lateral direction (Y). The antenna system comprises antenna devices being interspaced and mounted in connection to one of the short sides or both the short sides and extending in the height direction (Z). Each of the antenna devices comprises a waveguide board.

TECHNICAL FIELD

The present invention refers to an antenna system for an airborne radarsystem. The antenna system comprises a dorsal unit having two opposinglong sides extending in a height direction and a longitudinal direction,and two opposing short sides extending in a lateral direction and theheight direction, and an upper side opposing a bottom side eachextending in the longitudinal direction and the lateral direction. Theantenna system comprises antenna devices.

BACKGROUND

In the field of radar devices for airplanes it is known to use a dorsalunit positioned on the airplane body and extending in the longitudinaldirection of the airplane, i.e. in the direction from the fore to theaft. The dorsal unit comprises a number of side looking antenna elementspositioned along the longitudinal direction of the dorsal unit for sidelooking purposes. One problem with the dorsal arrangement is that theradar cannot see in a forward or rearward direction without additionalantenna elements being placed in the front and the rear of the dorsalunit.

The prior art document U.S. Pat. No. 5,923,302 concerns an endfire arraywith monopoles on the roof of the dorsal unit. Problems with thissolution are that it is limited in terms of antenna performance,expensive in terms of a complicated electromagnetic design process, anintricate scan control and complicated manufacturing. Furthermore, thesolution results in an undesirable upwards lobe tilt unless the groundplane is bent downwards towards the ends of the dorsal unit.

In prior art is also known to use a separate antenna in the nose of theaircraft for forward looking and an antenna inside a bulbous radomesomewhere at the aft for rearward looking. The solution to equip theseantennas with extra radar systems has the disadvantage of being costly.

Alternatively, a disadvantage with the forward and rearward lookingantennas connected to a common radar is that long high-power RF feedsmust be drawn from the radar to the forward/rearward looking antennas.This solution becomes unnecessarily heavy and it blocks the possibilityto install other, important sensors in the nose radome. Therefore, alightweight solution that utilizes the power delivered by the existingT/R-units is to be preferred.

There is thus a need for an antenna solution in a radar system providingfull coverage (360°) with no moving parts, minimized drag, minimizedweight, minimized system size, low cost, high gain and an electronicscan capability, and an overall improvement of the performance of theantenna system in a radar system with regard to forward and/or rearwardlooking abilities.

SUMMARY

The present invention refers to an antenna system for a radar system foran airplane. The antenna system comprises a dorsal unit extending in alongitudinal direction, a lateral direction and a height direction. Thedorsal unit comprises two opposing long sides extending in a heightdirection and a longitudinal direction, and two opposing short sidesextending in a lateral direction and the height direction, and an upperside opposing a bottom side each extending in the longitudinal directionand the lateral direction. The longitudinal direction coincidesessentially with the longitudinal direction of the airplane when thedorsal unit is mounted onto the airplane. The dorsal unit for a sidelooking radar system comprises a number of first antenna elementspositioned on each of the long sides of the dorsal unit and in thelongitudinal direction of the dorsal unit. The antenna system comprisesa microwave power distribution system comprising a number of firsttransmit-receive units (hereinafter called T/R-units) positioned insidethe dorsal unit and arranged to feed microwave power to and from thefirst antenna elements. The power distribution system arranged todistribute microwave power.

The present invention is characterized in that the antenna systemcomprises an assembly of interspaced antenna devices mounted inconnection to one of the short sides or both the short sides andextending in the height direction, wherein each of the antenna devicescomprises a waveguide board. By covering, and preferably extending thefront and/or aft projection of the dorsal unit with a number of planarwaveguide boards a beneficial high antenna performance is achievedwithout introducing excessive aerodynamic drag. The waveguide boardshave an extension essentially in the height direction and may extendvertically or may extend at an angle to a vertically extending line orplane. The antenna devices are preferably separated by a distancedetermined by the requirement that the forward and rearward lookingantennas are able to scan in the forward and rearward sectors in such away that grating lobes do not occur.

Another advantage is that the antenna system may be designed in such away that the antenna devices may be fed microwave energy by use of thefirst T/R-units already comprised in the dorsal unit. The antennadevices may thus transmit the entire power produced by all first T/Rmodules along the dorsal unit for utilizing all microwave power for ahorizontal scan in the fore and/or aft direction. Hence, the solutiondoes not require extra T/R modules why the assembly of antenna devicescan be mounted onto an already existing dorsal unit without a majorre-design of the dorsal unit.

The waveguide board also comprises second antenna elements coupled tosecond waveguides comprised in the waveguide board and arranged todistribute the power to/from at least one summation point to the secondantenna elements.

The first T/R-units are controlled by a control unit in such a way thatthe waveguide boards are fed microwave energy independently of the otherwaveguide boards for control of phases and amplitudes between differentantenna devices.

In one embodiment of the invention each antenna device comprises anaerodynamic housing encasing the waveguide board. One advantage of usingan aerodynamic housing is that fore and/or aft scanning can be achievedaccording to above with a minimum increase of aerodynamic drag.

The housing may comprise a foam-like material surrounding the waveguideboard. The foam-like material should be form stable high speedconditions and should at the same time be lightweight. The material isadvantageously hard, withstands mechanical stress, is lightweight, haslow electrical losses, and has advantageously a relative electricpermittivity close to one.

The use of a foam-like material gives the advantage of a low-cost andlightweight antenna device with high aerodynamic performance withoutadding much weight to the dorsal unit weight.

The housing may comprise a surface layer encasing the foam-like materialfor environment protection. The surface layer may comprise a metallicskin providing a ground plane for achieving an enhanced directivity,wherein the surface layer comprises slots in the height direction, i.e.in the vertical direction.

The antenna device may also comprise an RF-transparent housingsurrounding the waveguide board. The second antenna element is comprisedwithin the housing and protruding from the waveguide board in thelongitudinal direction for directivity purposes. Here “longitudinal”refers to a direction coinciding with the longitudinal direction whenthe dorsal unit is mounted onto an airplane. The second antenna elementmay also comprise a passive or active structure.

The antenna system may comprise a support structure for attachment ofthe antenna devices to the dorsal unit and for keeping the antennadevices in position relative the dorsal unit. The support structure maycomprise third waveguides for feeding microwave power to the waveguideboards via a connector.

In one embodiment the microwave power distribution system mayadvantageously comprise an assembly of polarized first waveguidesmounted on top and/or on the bottom of the dorsal unit and/or inside thedorsal unit. The antenna devices are connected to the first waveguidesso that the microwave power supplied by the first PR-units can bedistributed in such a way that an azimuthal scan can be performed by theradar system in the forward and/or the rearward sectors. The scan ismade by controlling the phases of the microwave power between theantenna elements by controlling the first T/R-units in a manner knownfrom prior art. The purpose of the invention is thus to allow scanningof a fore and/or aft lobe without using an antenna in the nose of theairplane and a radome at the tail of the same, or to use the also lesssatisfactory solution of the above described end fire solution describedin U.S. Pat. No. 5,923,302.

One benefit of the embodiment is that the first waveguide assembly canbe designed and manufactured at a low cost. Further advantages are thatit is less expensive and more lightweight than the nose and/or aftantenna known from prior art. A further advantage with the firstwaveguides is that the integration into the aircraft becomes simpler toperform.

Furthermore, the following advantages are shared with the antenna systemdescribed in U.S. Pat. No. 5,923,302, namely, that the weight does notadd significantly to the dorsal unit weight, and that the firstwaveguide assembly can be mounted onto the dorsal unit without majorre-design of an existing dorsal unit. Yet furthermore, since the firstwaveguides extend essentially over the entire length of the dorsal unit,the first waveguides may be used for feeding energy to both the fore andaft antenna elements. Still furthermore, the first waveguides and theantenna devices form a collection of parts that may easily be mounted insitu directly onto the existing dorsal unit and interconnected to eachother and connected to the already existing devices, for example thefirst T/R-units. The first T/R-units may be coupled to the firstwaveguides by connecting all first T/R-units to a dedicated firstwaveguide and to equip adjacent first waveguides with apertures forallowing the electromagnetic signal in the dedicated first waveguide tothe remaining first waveguides. In an alternative embodiment oneT/R-unit is coupled to one first waveguide and the number of firstwaveguides and T/R-units are the same. In a yet further embodiment, afew T/R-units, say N_(T/R) are coupled to each of the N_(FWG) firstwaveguides so that. N_(T/R) multiplied with N_(FWG) approximately equalsN_(T/R,TOT). Since N_(FWG) is equal to N_(AD), the number of antennadevices, this approximate relation could also be expressed as:N_(T/R)=N_(T/R,TOT)/N_(AD)).

However, the present invention has the following advantages over thedevice described in U.S. Pat. No. 5,923,302; it is inexpensive in termsof the electromagnetic design process, it does not need an intricatescan control, and does not need complicated manufacturing. Furthermore,the present invention does not result in an undesirable upwards lobetilt. Yet furthermore, the present invention may be designed for abetter and more controllable antenna performance in terms of lobe widthsand scannability.

The assembly may have a planar extension in the lateral direction butmay also have a somewhat dome shaped or curved cross-section, but mayalso be arranged in a staggered manner, i.e. in a zigzag pattern where anumber of first waveguides being partly or fully on top of other firstwaveguides.

In one embodiment the second antenna elements are connected to secondT/R-units being arranged to be controlled in such a way that themicrowave power supplied by the first T/R-units can be distributed bythe antenna system in such a way that an elevation scan is performed bythe radar system in a direction out from one of the short sides or boththe short sides, i.e. in the forward direction and/or the aft directionof an airplane when the dorsal unit is mounted onto the airplane.

The antenna system according to the invention may thus be used for a360° azimuthal scan in a plane described by the lateral direction andthe longitudinal direction by use of a control unit for controlling thefirst T/R-units to feed microwave energy to the first antenna elementsand the second antenna elements respectively with a phase increment inthe plane. The first T/R-units may thus comprise a switch devicecontrolled by the control unit for controlling the feed of microwaveenergy to the first antenna elements or to the second antenna elementsdepending on the direction of the scan. The first antenna elements areused for essentially a lateral scan on both sides of the dorsal unit andthe second antenna elements are used for forward and rearward scan. Theantenna system may also perform an elevation scan by controlling thesecond T/R-units to feed microwave energy to the second antenna elementswith a phase increment in the height direction.

The stated advantages and embodiments will become apparent in thedetailed description of the invention below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given herein below in the accompanying drawingswhich are given by way of illustration only, and thus are not limited tothe present invention and wherein:

FIG. 1 schematically shows an embodiment of a microwave powerdistribution system in an antenna system according to the invention foran airborne radar system;

FIG. 2 schematically shows a microwave power distribution systemaccording to the invention;

FIG. 3 schematically shows a front view of an antenna system accordingto the invention;

FIG. 4 schematically shows a side view of a waveguide board according tothe invention;

FIG. 5 a schematically shows an antenna device according to a firstembodiment of the invention;

FIG. 5 b schematically shows an antenna device according to a secondembodiment of the invention;

FIG. 6 schematically shows a cross-section of an antenna deviceaccording to a third embodiment of the invention;

FIGS. 7 a and 7 b schematically show a connector for the waveguide boardaccording to the invention;

FIG. 8 a schematically shows a front view of an antenna system accordingto the invention comprising a radome, and wherein;

FIG. 8 b schematically shows a top view of an antenna system accordingto FIG. 8 a.

DETAILED DESCRIPTION

FIG. 1 schematically shows an antenna system 1 for an airborne radarsystem according to the invention. The antenna system 1 comprises anumber of first antenna elements 2 and a microwave power distributionsystem 3 comprising a number of first T/R-units 4 arranged to distributemicrowave power to and from a microwave receiver and generator 5 to thefirst antenna elements 2. FIG. 1 schematically teaches an advantageousembodiment of the microwave power distribution system 3. In FIG. 1 thepower distribution system 3 comprises a planar assembly of polarizedfirst waveguides 6 coupled to a number of second antenna elements 7directed at an angle essentially perpendicular to the number of firstantenna elements 2. The first T/R-units 4 are arranged for distributionof microwave energy to the first and second antenna elements 2, 7.

The antenna system comprises a dorsal unit 8 having two opposing longsides 9 extending in a height direction Z and a longitudinal directionX, and two opposing short sides 10 extending in a lateral direction Yand the height direction Z, and an upper side 11 opposing a bottom side12 each extending in the longitudinal direction X and the lateraldirection Y. The directions X, Y and Z are only mentioned in order tofacilitate the description and understanding of the invention and are inFIG. 1 depicted as an orthogonal system. It should be noted that thedorsal unit does not have to be a rectangular box, but may comprisesides having a non-planar extension. For example, the upper side 11 mayhave a somewhat dome shaped or curved cross-section taken in the lateraldirection. The first waveguides 6 may then follow the shape of the upperside 11 or may be arranged with a different contour.

In FIG. 1 the assembly of polarized first waveguides 6 is mounted on topof the dorsal unit 8, i.e. on the upper side 11 of the dorsal unit 8.The assembly of first waveguides 6 may alternatively be positioned atthe bottom side 12 of the dorsal unit 8 or within the dorsal unit 8. InFIG. 1 the dorsal unit 8 is mounted onto the top of an airplane 13 sothat the longitudinal direction of the airplane 13 coincides with thelongitudinal direction of the dorsal unit. The airplane 13 is left outin the remaining drawings in order to minimize the number of features inthe drawings. However, the dorsal unit 8 is intended to be mounted ontoa device moving at high speed, preferably in the air. The high speedfeature is of importance since it puts high demands on the antennasystem with regard to aerodynamical features such as aerodynamic drag aswell as elevated temperatures and wear due to, for example, rain andsand erosion.

In FIG. 1 the first antenna elements 2 are positioned at leastlongitudinally on each of the long sides 9 of the dorsal unit 8 and thesecond antenna elements 7 are positioned in connection to one of theshort sides 10 or both the short sides 10. FIG. 1 shows antenna devices14 according to the invention mounted onto both the short sides 10 ofthe dorsal unit 8. The antenna devices 14 comprise the second antennaelements 7 which are connected to the first, waveguides 6. The antennadevices 14 will be described further below.

In FIG. 1 the first T/R-units 4 are positioned within the dorsal unit 8,but they may be positioned at a location outside the dorsal unit 8, forexample inside the airplane 13. The first T/R-units 4 are often referredto as T/R-modules and they serve the purpose of feeding RF signals fromthe microwave generator 5 to the antenna elements 2, 7 during atransmission period, and receiving RF signals from the antenna elements2, 7 while switching off the energy feed during a receiving period.During a first transmission period the first T/R-units 4 feed energyfrom the power source to the first antenna elements 2 via for example agalvanic coupling or by use of a contact-less electromagnetic coupling.During a second transmission period the first T/R-units 4 feed energyfrom the power source to the second antenna elements 7 via the firstwaveguides 6. The energy fed from the first T/R-units 4 to the firstwaveguides 6 may be done by any suitable means, for example by use of agalvanic conductor, i.e. a flexible cable or the like, connecting aT/R-unit 4 with a transition device for transforming the electricalsignal into an electromagnetic microwave signal. The microwave signal isfed by the transition device into the first waveguides where themicrowave signal propagates in a known manner. The energy from theT/R-units 4 may also be fed to the first waveguides by means of acontact-less electromagnetic coupling.

During the receiving period the antenna elements 2, 7 receive returningelectromagnetic power previously sent out having been reflected from anobject, for example a target. During the listening period the microwavepower distribution system 3 comprises means for feeding the returningmicrowave energy to receivers for signal processing in the radar system.With regard to the second antenna elements 7, the first waveguides 6 areconstructed to feed the returning microwave energy directly to thereceivers or to a converting device converting the electromagneticsignal in the first waveguides 6 into an electric signal in a cable forfurther feeding the microwave energy to the receivers.

In FIG. 1, the distribution system 3 comprises an assembly ofhorizontally polarizing rectangular first waveguides 6 but may be in theform of vertically polarizing first waveguides or circularly polarizingfirst waveguides or first waveguides polarized in any other suitableway. The first waveguides 6 are not limited to a rectangularcross-section, but may have any geometric cross-section suitable forguiding microwaves, for example circular, oval, and ridged.

FIG. 2 schematically shows a microwave power distribution system 3according to the invention. The first waveguides 6 in FIG. 1 are notvisible in FIG. 2 and it should be noted that the invention is notrestricted to the use of the first waveguides 6 in FIG. 1. Hence, thefirst waveguides may be replaced by any suitable means for feeding amicrowave signal. However, the use of first waveguides is advantageousbecause of the easier assembly of the antenna system and for thelightweight construction and for the other reasons stated above.

The first waveguides 6 may each be fed the microwave signal by use of atransforming device comprising a probe being either magnetic, electricor adapted to transmit/transform energy in any other suitable way.However, a vast amount of first waveguide feeding techniques are knownfrom prior art which may be applied on the invention.

The feeding of energy to the first waveguides 6 needs to be controlledin order to control the phase shifts in the first waveguides 6 in orderto direct the energy in the front or aft direction of the airplane.Hence, the phase increment of the T/R-units needs to be set in order tohave a constructive adding of energy in the desired direction.Therefore, a control device (not shown) is comprised in the antennasystem for control of the first and the second antenna elements 2, 7.

According to one embodiment of the invention one dedicated firstwaveguide 6 may be fed microwave energy by all first T/R-units 4. Themicrowave power is distributed to the adjacent first waveguides 6 viaapertures (not shown). The waves are distributed from the dedicatedfirst waveguide 6 a in the lateral direction Z towards the mostperipheral first waveguides. The signal propagation is intended to be inthe longitudinal direction of the first waveguide assembly, i.e. in thedirection from end to end, and is utilized at the end sections only.

The dedicated first waveguide 6 may be the central first waveguide orany other first waveguide. The distribution system 3 comprises twolinear assemblies of phase shift devices (not shown) arranged at eachend of the first waveguide assembly 6. One phase shift device ispositioned at each end of each first waveguide 6. The phase shiftdevices may be of any type known from prior art, for example ferritephase shift devices. The phase shift devices may be mounted onto the endof the first waveguide 6 or may be inserted into an end part of thefirst waveguide 6.

The advantages of this embodiment are a highly modular and low-costdesign. For instance, only one feed transition, for example the abovedescribed probe, between the first waveguide 6 assembly and each of thefirst T/R-units 4 needs to be designed. It does not offer thepossibility of scanning of the fore/aft lobe without using a linearassembly of additional phase shifters at the first waveguide 6 endsbecause the phase in one first waveguide will be determined by thephases of the neighboring feed first waveguide. However, the embodimenthas the advantage that the entire first waveguide assembly 6 may bemanufactured separately from the dorsal unit 8 and may then easily bemounted onto an already existing dorsal unit and connected to thealready existing first T/R-units 4 by simple means.

The antenna devices according to the invention allow for an azimuthalscan in the X-Y-plane. The lobes extend essentially in the forwarddirection X and scanning is performed in the lateral direction Y. Theazimuthal scan is created by use of the second antenna elements beingfed microwave power via the first waveguides 6. The azimuthal scan hasbeen created by use of the control device for controlling the firstT/R-units 4 according to above and the phase shift devices. The secondantenna elements 7 are arranged to cover the forward sector, and inappropriate oases the aft sector, which cannot be scanned by the firstantenna elements. The first antenna elements 2 may be used to scan asector being 2 times an angle α (2×α) and the second antenna elements 7may be used to scan a sector being 2×(90°−α). Here, the angle α refersto an angle between a normal extending in the lateral direction Y, i.e.in a direction being essentially perpendicular to the longitudinaldirection X of the dorsal unit 8, and a tangent in the longitudinaldirection X. The antenna system 1 may thus be used to scan 360° in theX-Y-plane. It can be mentioned as an example that if the first antennaelements cover a sector of 120° i.e. 2 times 60° on each side of thedorsal unit 8 and the second antenna elements cover a sector of 60°,i.e. 2 times 30° in both the fore and aft direction.

According to another embodiment the first T/R-units 4 feed all firstwaveguides 6. The fact that the feed points of a first waveguide mustobey certain phase relationships for efficient propagation does notprohibit that the phases between the first waveguides 6 can be givenarbitrary values. Hence, fore and/or aft scanning is possible withoutextra phase shift devices. This solution avoids the costs and weightassociated with phase shift devices. However, the phases of the firstT/R-units 4 have to be flexibly controlled by the control device inorder to be able to control the direction of propagation of themicrowave signal in the cluster of first waveguides 6. The controldevice therefore controls the first T/R-units 4 according to a selectedalgorithm giving the control of the direction of propagation.

The first waveguide assembly 6 may be manufactured separately in thesame manner as the first waveguide assembly described in connection thefirst embodiment. One difference however between the two embodimentsdescribed above is that the latter embodiment has to have feedtransitions to all first waveguides, for example by the above describedprobe. However, since the phase shift devices are not necessary, theembodiment also has the advantages of being of a highly modular andlow-cost design.

Also in the latter embodiment the first and second antenna elements 2, 7are positioned so that the antenna system 1 can be controlled to cover a360° azimuthal scan by alternating between the first antenna elements 2and the second antenna elements 7.

It should be noted that each of the first T/R-units 4 are directlycoupled to the side looking first antenna elements 4 on each long sideof the dorsal unit 8, but that the first T/R-units 4 are indirectlycoupled to the second antenna elements 7 via the first waveguides 6.Since at least a number of the first T/R-units 4 are switched to anumber of the first waveguides 6, the phases between the first T/R-unitsmay be controlled so that the common signal from the first T/R-units arefed in the fore or aft direction in the first waveguides 6. Hence, thefirst T/R-switches may be controlled so that the antenna system mayperform a scan on all sides of the dorsal unit 8.

FIG. 3 schematically shows a front view of an antenna system accordingto the invention. The antenna system 1 comprises antenna devices 14mounted in connection to one of the short sides 10 or both the shortsides 10 and extending in the height direction Z. The antenna devices 14are preferably positioned essentially parallel to each other with aselected distance D1 between them. The selected distance D1 may bedecided dependent on the desired performance of the antenna system 1 andon minimizing the aerodynamic drag. It should be noted that it is thecenter-to-center distance that relates to the desired performance andthat the distance D1 in relation to the center-to-center distance thatrelates to drag. The second antenna elements 7 are comprised in theantenna devices 14 and are preferably positioned in each of the antennadevices 14 in a row, i.e. in a series after each other in the heightdirection Z.

The antenna devices 14 may be mounted directly onto the short side(s) 10or may be mounted to the dorsal unit via brackets 15. The antennadevices 14 may also be interconnected via brackets 15 such that theantenna devices form a separate unit easily mounted onto an alreadyexisting dorsal unit. The antenna devices 14 are connected to the firstwaveguides 6 by any known means, for example by contact-less connectormeans or galvanic connector means. The number of antenna devices 14 iscorrelated to the number of first waveguides in such a way that there isone antenna device connected to each first waveguide 6. One advantage ofusing the antenna devices 14 is that the effective antenna aperture areais increased at the same time as the aero dynamic drag is kept to aminimum. The increased effective antenna aperture area gives thepossibility of increased gain and thus the possibilities to create morenarrow lobes for better detection of targets.

Furthermore, the antenna devices 14 are connected to the firstwaveguides 6 so that the microwave power supplied by the first T/R-units4 can be distributed by the antenna system in such a way that anazimuthal scan according to the above is performed by the radar systemin a direction out from one of the short sides 10 or both the shortsides 10, i.e. in the forward direction and/or the aft direction of anairplane when the dorsal unit is mounted onto an airplane.

In a further embodiment the second antenna elements 7 are connected tosecond PR-units 16 positioned between the first waveguides 6 and thesecond antenna elements 7. The second T/R-units 16 are arranged to becontrolled by the control unit. The microwave power supplied by thefirst T/R units 4 is fed to the second T/R-units 16 via the firstwaveguides 6. The second T/R-units 16 are controlled by the control unitin such a way that the phase increment between the second antennaelements 7 gives an elevation scan in a direction out from one or boththe short sides 10, i.e. in the forward direction and/or the rearwarddirection of an airplane when the dorsal unit is mounted onto anairplane. The antenna system 1 may thus use the first T/R-units 4 for anazimuthal scan and the second T/R-units for an elevation scan.

The above described scans are made by controlling phases in differentantenna elements by the control of the first and/or the second T/R-units16 in a manner known from prior art and will not be explained further.

The antenna devices 14 may be realized in a number of different ways.For example, each antenna device 14 comprises a layered structurecomprising in the lateral direction an electrically conducting layer 17onto a non-conducting 18 layer positioned adjacent a number of secondantenna elements 7 and on the other side of the antenna elements 7 asecond non-conducting layer 19 covered with a conductive layer 20. Thesize of the antenna devices 14 is dependent on the intended use of theantenna system, i.e. the intended use of the radar system that comprisesthe antenna system.

Below is an example of an antenna device suitable for an airborne S-bandradar: The measurements are 10 mm times 100 mm times the height whichmay be less than, equal to or greater than the height of the dorsalunit. The antenna devices are separated by a selected distance D1=70-80mm depending on a number of parameters, for example the wavelength ofthe microwave transmitted. The separation therefore has to be calculatedwith regard to these parameters.

The antenna devices 14 form an assembly of antenna devices 4 forming anantenna. One benefit of using such thin antenna devices 14 in theproposed manner is that the antenna may extend outside the dorsal unitin the lateral direction without significant increase of aerodynamicdrag. The possibility to extend the cross-section of the antenna systemin the forward and/or aft direction is a major benefit of the inventionsince the more antenna devices and the wider the antenna system is inthe lateral direction, the narrower n the lobe be formed.

Further advantages of the invention are that the dorsal fin antennaassembly is thin, light and requires no moving parts, and thusadvantageously replaces the previously known AWACS rotodome typeantenna.

FIG. 4 schematically shows a side view of a waveguide board 21 accordingto the invention. Each antenna device 14 comprises at least onewaveguide board 21 and each of the waveguide boards 21 are fed microwavepower independently of the others in order to allow the antenna system 1to be phase-steered.

Each waveguide board 21 comprises second waveguides 22 that distributevertically Z the power to/from typically one or two summation points toa number of, typically 10-20, second antenna elements 7. FIG. 4 shows anexample of a waveguide board 21 with one summation point SP. The planarwaveguide board 21 can be realized in different techniques, where asuspended stripline is a beneficial choice because of its low losses.The waveguide board 21 comprises at least one connector 30 for waveguide transition from, for example, the above described assembly offirst waveguides 6 or from third waveguides (see FIG. 7) to the secondwaveguides 22. The third waveguides 29 connects the waveguide board 21to the distribution system (3 in FIGS. 1-3). As been stated above, thedistribution system 3 advantageously comprises the assembly of firstwaveguides 6, but may comprise an alternative microwave feeding device,for example a flexible cable or the like. This is of course a completelydifferent solution where the advantages of using the waveguides 6 arelost.

Because of the lack of a ground plane, the antenna elements shouldpossess an inherent directivity. There are several known types ofelements that fulfill this requirement. The below described FIGS. 5 a, 5b and 6 all show different embodiments for an antenna device accordingto the invention where the ground plane has been established in threedifferent ways. In all embodiments shown in FIGS. 5 a, 5 b, 6, thewaveguide board 21 is positioned within a housing 23 comprising afoam-like material 24.

FIG. 5 a schematically shows antenna device 14 according to a firstembodiment of the invention. In FIG. 5 a the antenna device comprises aground plane 26 comprised within the housing 23 adjacent the waveguideboard 21 and coupled to the second waveguides (22, FIG. 4). The secondantenna element 7 is comprised within the housing 23 and protrudes fromthe ground plane 26 in the longitudinal direction X for directivitypurpose.

FIG. 5 b schematically shows an antenna device 14 according to a secondembodiment of the invention. In FIG. 5 b the antenna devices 14 aresimilar to the antenna devices according to FIG. 5 b, but with theexception that the second antenna element is a passive or activestructure for directivity purpose.

FIG. 6 schematically shows a cross-section of an antenna device 14according to a third embodiment of the invention. Only a part of theantenna device 14 is shown in FIG. 6. In FIG. 6 the housing comprises asurface layer 25 encasing the foam-like material 24 for increased formstability. The surface layer 25 may comprise a metallic skin providing aground plane 26 for achieving an enhanced directivity. The surface layer25 comprises slots 28 in the height direction Z, i.e. in the verticaldirection, in connection to the second antenna elements 7.

In FIG. 6 the planar waveguide board 21 feeds the vertical slots 28 inthe conductive surface layer 25 via the second antenna element 7. Themetallic skin provides the ground plane needed for achieving an enhanceddirectivity.

FIGS. 7 a and 7 b schematically show third waveguides 29 in a supportstructure 15 according to the invention. In FIG. 7 the support structure15 is coupled to the antenna devices 14 for keeping the antenna devices8 in position relative the dorsal unit (not shown in FIGS. 7 a and 7 b).The third waveguides 29 are arranged for feeding microwave power to thewaveguide boards 21 via the connector 30.

FIG. 7 a shows a segment of the third waveguide 29 coupled to theantenna device 14. In FIG. 7 a the third waveguide 29 has a rectangularcross-section and is arranged for feeding a horizontally polarizedsignal. In FIG. 7 a the connector 30 comprises a horizontally orientedslot 31 for receiving the signal being fed to the connector by the thirdwaveguide 29. In the area where the third waveguide 29 meets theconnector 30, the third waveguide has a vertical extension. Thehorizontal slot 31 is thus arranged essentially perpendicular to theextension of the third waveguide 29.

FIG. 7 b shows a segment of the third waveguide 29 coupled to theantenna device 14. In FIG. 7 b the third waveguide has a rectangularcross-section and is arranged for feeding a vertically polarized signal.In FIG. 7 b the connector 30 comprises a vertically oriented slot 32 forreceiving the signal being fed to the connector by the third waveguide.In the area where the third waveguide 29 meets the connector 30, thethird waveguide has a horizontal extension. The vertical slot 32 is thusarranged essentially perpendicular to the extension of the thirdwaveguide 29.

In another embodiment the third waveguides 29 in FIGS. 7 a and 7 b maybe the first waveguides 6. The arrangement in FIGS. 7 a and 7 b may thusbe applied on an arrangement where the third waveguides are replaced bythe first waveguides. In all embodiments it is possible to use extrasupport structures.

FIG. 8 a schematically shows a front view of an antenna system accordingto the invention comprising a radome 33. In order to reduce aerodynamicdrag, the dorsal unit 8 may be partly covered with a radome 33 forcovering the front projection of the dorsal unit 8. Since the antennadevices 14 may have a larger vertical extension than that of the dorsalunit 8, the antenna devices 14 that are in fore and/or in the aft of thedorsal unit 8 may protrude out from the radome 33.

FIG. 8 b schematically shows a top view of an antenna system accordingto FIG. 8 a. Tight separation of the feed waveguides may make itdifficult to bend them onto the planar waveguide boards 21. However, arealization may be facilitated by (i) separating the antenna/waveguideboards to the grating lobe limit imposed by ±30 degree scannability,(ii) realizing the feeding second waveguides 22 as narrow and ridgedwaveguides. Space is then made free for curving the second waveguidesonto the boards. The support structure 15 therefore widens towards theantenna devices 14, as seen in FIG. 8 b, in order to house the spreadingsecond waveguides 22.

1-24. (canceled)
 25. An antenna system for an airborne radar system, theantenna system comprising: a dorsal unit having two opposing long sidesextending in a height direction (Z) and a longitudinal direction (X) andtwo opposing short sides extending in a lateral direction (Y) and theheight direction (Z), and an upper side opposing a bottom side eachextending in the longitudinal direction (X) and the lateral direction(Y), wherein the antenna system further comprises a plurality of firstantenna elements interspaced and mounted in connection to one or both ofthe short sides and extending in the height direction (Z), wherein eachof the first antenna elements further comprises a waveguide board. 26.The antenna system according to claim 25, wherein the waveguide boardfurther comprises second antenna elements.
 27. The antenna systemaccording to claim 26, wherein each waveguide board comprises secondwaveguides arranged to distribute the power to/from at least onesummation point to the second antenna elements.
 28. The antenna systemaccording to claim 27, wherein each of the waveguide boards are arrangedto be fed microwave energy independently of the other waveguide boardsfor control of phases and amplitudes between different antenna elements.29. The antenna system according to claim 25, wherein each waveguideboard is comprised within an aerodynamic housing.
 30. The antenna systemaccording to claim 29, wherein the housing comprises a foam-likematerial surrounding the waveguide board.
 31. The antenna systemaccording to claim 30, wherein the housing comprises a surface layerencasing the foam-like material.
 32. The antenna system according toclaim 31, wherein the surface layer is a metallic skin providing aground plane for achieving an enhanced directivity, wherein the surfacelayer comprises slots in the height direction (Z).
 33. The antennasystem according to claim 29 wherein the antenna device comprises aground plane within the housing adjacent the waveguide board and coupledto the second waveguides, wherein the second antenna elements are withinthe housing and protruding from the ground plane in the longitudinaldirection (X) for directivity purposes.
 34. The antenna system accordingto claim 25, wherein the second antenna elements comprise a passive oractive structure for directivity purposes.
 35. The antenna systemaccording to claim 25, wherein the antenna system comprises a supportstructure comprising third waveguides for feeding microwave power to thewaveguide boards via connectors.
 36. The antenna system according toclaim 35, wherein the connector is vertically oriented or horizontallyoriented.
 37. The antenna system according to claim 25, wherein theantenna elements are separated so that a grating lobe limit is imposedby a prescribed, limited scannability.
 38. The antenna system accordingto claim 25, wherein each waveguide board is planar.
 39. The antennasystem according to claim 26, further comprising a microwave powerdistribution system having a number of first transmit/receive (T/R)units being coupled to the first antenna elements for distribution ofmicrowave power to the first antenna elements, wherein the microwavepower distribution system is coupled to the second antenna elements. 40.The antenna system according to claim 39, wherein the microwave powerdistribution system further comprises an assembly of first waveguidescoupled to the second antenna elements being directed at an angleessentially perpendicular to the first antenna elements, wherein thefirst T/R units are coupled to the first waveguides for distribution ofmicrowave energy to the second antenna elements.
 41. The antenna systemaccording to claim 39, wherein the assembly of first waveguides aremounted on the upper side and/or on the bottom side of the dorsal unitand/or inside the dorsal unit.
 42. The antenna system according to claim39, wherein the first antenna elements are positioned at leastlongitudinally (X) on each of the long sides of the dorsal unit and thatthe second antenna elements are positioned in connection to one or bothof the short sides.
 43. The antenna system according to claim 39,wherein the antenna devices are connected to the first waveguides sothat the microwave power supplied by the first T/R units can bedistributed by the antenna system such that an azimuthal scan isperformed by the radar system in a direction out from one or both of theshort sides, in the forward direction and/or the rearward direction ofan airplane when the dorsal unit is mounted onto an airplane.
 44. Theantenna system according to claim 39, wherein the second antennaelements are connected to second T/R units being arranged to becontrolled such that the microwave power supplied by the first T/R unitscan be distributed by the antenna system such that an elevation scan isperformed by the radar system in a direction out from one or both of theshort sides in the forward direction and/or the aft direction of anairplane when the dorsal unit is mounted onto the airplane.
 45. Theantenna system according to claim 39, wherein the first antenna elementsare connected to the first T/R units so that the microwave powersupplied by the first T/R units can be distributed by the antenna systemin such a way that an azimuthal scan is performed by the radar system ina direction out from one or both of the long sides in the lateraldirection (Y) of an airplane when the dorsal unit is mounted onto theairplane.
 46. The antenna system according to claim 39, furthercomprising a control device for controlling the first T/R units andthereby the phase shifts of the microwave power between the firstantenna elements and the second antenna elements.
 47. The antenna systemaccording to claim 46, wherein the first antenna elements are positionedessentially parallel to the second antenna elements.
 48. The antennasystem according to claim 46, wherein the second antenna elements arepositioned in series in the height direction.